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UNIVERSITY OF EAST ANGLIA
School of Computing Sciences
May/June UG Examination 2006-2007
COMPUTER NETWORKS
CMPS3D22
Time allowed: 3 hours
Answer THREE questions.
All questions carry equal weight.
Notes are not permitted in this examination.
DO NOT TURN OVER UNTIL YOU ARE TOLD TO DO SO BY THE
INVIGILATOR
(CMPS3D22)
© Copyright of the University of East Anglia
-21. (a) A commonly used model of the functionality of a network architecture is that of the
OSI seven layer reference model.
(i)
Name and explain the functionality provided by each layer of the OSI seven
layer reference model.
(ii)
[7 marks]
Explain how layered network architectures operate and in so doing how they
are able to reduce design complexity. In particular discuss the role of layers,
interfaces, protocols, encapsulation and peer processes.
(iii)
[9 marks]
Explain why the OSI seven layer reference model contains both a transport
layer and a network layer.
[4 marks]
(b) Consider the small network shown in Figure 1.
4
2
v3
v1
v6
4
3
v4
4
2
2
3
v2
v5
Figure 1
(i)
Describe the internal structure of switching node v2 for this network, explaining
the role and contents of each data structure you identify. Explain in detail how
v2 routes a packet it receives from v5.
(ii)
[6 marks]
Explain what is meant by ‘static routing’ and ‘adaptive routing’, and discuss the
advantages and disadvantages of each.
[4 marks]
Question 1 is continued on page 3
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-3(iii) Describe the basic operation of one adaptive routing technique. With reference
to the network of switching nodes shown in Figure 1, illustrate how this routing
strategy is ‘adaptive’ to loss of the link between v2 and v4.
[10 marks]
2. (a) The Selective Repeat protocol is a sliding window protocol that ensures reliable
delivery of packets over an unreliable communication channel. Explain the operation
of Selective Repeat and in particular comment on the use of send and receive
windows, sequence numbers and acknowledgement packets.
[8 marks]
(b) Within the Selective Repeat protocol explain, with the help of illustrations, the effect
of losing
(i)
A message packet
(ii)
An acknowledgment packet
and show how recovery from these events takes place.
[12 marks]
(c)
A local Area Network (LAN) Medium Access Control (MAC) protocol aims to
achieve fair and efficient access to a LAN transmission medium.
(i) Characterise the distinction between permission based and contention based MAC
layer protocols and discuss the general properties of each for addressing the issues
of fairness and efficient access for light and heavy loading of each kind of
protocol.
[8 marks]
(ii) The IEEE 802.11 Medium Access Control (MAC) protocol provides for both
permission based and a contention based mode of operation. Describe in detail
the two different modes of operation supported by IEEE 802.11 and how the
protocol design allows these modes to coexist.
[12 marks]
(CMPS3D22)
TURN OVER
-43. (a)(i) Explain the operation of a block interleaver and show how the interleaver is able to
reduce the effect of burst-like packet loss when transmitting packets across an
unreliable network.
[8 marks]
(ii) The interleaving function of a block interleaver is given as
(id + j) = (d – 1 – i) + jd
where 0 ≤ i,j ≤ d-1
For an interleaving depth of d=2, compute the output sequence of the interleaver.
[8 marks]
(iii) Explain what is meant by the delay and spread of an interleaver. Calculate the delay
and spread of the interleaver defined above.
[4 marks]
(b)
“Congestion is the most critical attribute which determines the ability of a network
to deliver adequate performance for real-time applications.”
Define the term ‘congestion’ and characterize the differences between open-loop
and closed-loop approaches to network architecture design which seek to alleviate
this problem. Illustrate your answer with specific detail from ONE type of network
protocol stack which is appropriate for addressing the needs of real-time
applications.
[20 marks]
(CMPS3D22)
-54. (a)(i) The data link layer of the OSI seven layer reference model can employ forward
error correction (FEC) or backward error correction (BEC) to reduce the effect of
errors that occur in transmission. Explain the operation and differences between
FEC and BEC and comment on their effectiveness at detecting bit errors over both
uni-directional and bi-directional channels.
[6 marks]
(ii) The cyclic redundancy check (CRC) is often used for backward error detection.
CRC involves dividing a generator polynomial into a message to determine the
remainder which subsequently provides redundant information to determine if an
error has occurred. Given a standard generator polynomial of 100 and a message
sequence of 11010, determine the remainder that forms that CRC.
[9 marks]
(iii) Within the CRC method of error detection, describe the information that is
transmitted to the receiver and explain how this information is used to detect bit
errors that may have occurred in transmission.
[5 marks]
(b)
Two separate intranets are internet-worked across a Wide Area Network (WAN).
Each intranet consists of a number of local area network (LANs) connected by layer 2
and layer 3 bridges/routers. Each intranet is connected to the WAN by its own layer 3
router. The intranets have IP addresses 178.18.0.0/16 and 130.12.0.0/16 respectively.
(i) Illustrate by a diagram this network configuration.
[6 marks]
(ii) State the purpose and describe the operation of the IP Address Resolution
Protocol (ARP). Explain during which period(s) this protocol is used if host
178.18.2.23 tries to send a packet to host 130.12.22.22 assuming that neither
host’s data link layer address is cached within local routers.
[10 marks]
(iii) During transmission through the WAN the packet’s payload is corrupted. Explain
what error recovery is involved if a sliding window protocol is used.
[4 marks]
(CMPS3D22)
TURN OVER
-65. (a)
The physical layer (layer 1) of the OSI seven layer reference model converts digital
data received from the data link layer (layer 2) into an analogue for that can be
transmitted over a physical medium. To represent digital data as an analogue signal
several different modulation techniques can be used. Identify three such modulation
methods and describe their operation.
[9 marks]
(b)
Describe the differences between connectionless and connection-oriented services.
Name a protocol that provides connectionless communication and another that
provides connection-oriented communication.
[11 marks]
(c)
A local area network is based upon a 2.5 Kilometre 10Mbs coaxial cable using the
IEEE 802.3 (Ethernet) protocol.
(i)
Describe in detail the operation of this local area network protocol, and in so
doing describe the frame format and explain the need for a minimum data frame
size on such networks.
[14 marks]
(ii)
Assume that N hosts attempt to transmit 10M bit files simultaneously across this
network, explain how the operation of the IEEE 802.3 network protocol
performs when N is 1, 2, and 20.
[6 marks]
END OF PAPER
(CMPS3D22)